The present invention is directed in at least some of its embodiments to a display with a light source, for example a solid state light source, such as e.g. a light-emitting diode (LED), as a backlight. Robustness, reliability and long life of LEDs are known to be advantageous. However, currently, the intensity output of some light sources, in particular of solid state light sources, such as LEDs, varies according to factors such as temperature and age. Consequently, conventional LED based backlights and others do not maintain desired intensity and/or colour during their lifetime. The present invention seeks to solve this problem.
In a typical multi-colour based backlight, e.g. RGB backlight, a plurality of optical sensors, e.g. 3 in the case of RGB backlight, are based in the backlight cavity. Each optical sensor is read out by a control device that compensates the drive settings to the correct or desired white point, based on the read out luminance values. Typically, the three optical sensors are placed in one package and have a spectral response as shown in FIG. 1. Because the colour filters of the optical sensors are overlapping, there is an influence of the other colours during readout of one colour. For example, if one reads out GREEN, also a part of RED and BLUE is in the end result, as shown in FIG. 1, in particular in the left hand side showing the non-integrated sensor values. It can be seen that, when RED is switched off while GREEN is still on, the red sensor will still sense some light, i.e. that part of the GREEN which is in the wavelength range detectable by the red sensor. In typical systems, the LEDs are driven by PWM, as shown in the top halve of FIG. 2, and sensor values are integrated to DC for measurements, as illustrated in the middle and right hand side of FIG. 2. This results in very slow response times and if high dimming ratio is required also in high resolution and expensive A/D converters being required. To avoid the effect of interference of other colours in the optical sensors, colour sequencing can be used, but this may result in colour break-up and lower dimming ratios.
An LED-based luminaire is known from WO 2006/014473, which includes an emitter module having one or more LEDs and a regulating device that regulates the current delivered to the emitter module. The luminaire may include an optical sensor that measures the LED radiant output, and a controller that uses the detected output to control the regulating device based on the measured output, in order to maintain a consistent colour and/or intensity level. The LED-based luminaire may incorporate one or more colour channels, and the optical sensor may produce an intensity output for each colour corresponding to the colour channels. The sensor may be a single integrated circuit device which is capable of detecting multiple colour channels. If such sensor has to sense the luminance of the different colour channels, typically each colour will be driven separately sequentially. A disadvantage of this method is that colour break-up will occur, and that therefore the refresh rate of a display with such LED-based luminaire as backlight needs to be very high, e.g. 600 to 700 Hz.